Rosetta mission: What is it and why will it crash into a comet today?

What is the Rosetta mission?

Rosetta is a ground-breaking £1 billion mission launched in 2004 to chase, orbit, and land on a comet. It's coming to an end today after 13 years.

The spacecraft spent 10 years chasing comet 67P/Churyumov-Gerasimenko almost 500 million miles from Earth, before landing a probe called Philae on it in November 2014.

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The probe's batteries ran out within three days, forcing it to go into hibernation. It has effectively been out of touch since then, although was revived briefly in June and July 2015 as the comet moved closer to the sun.

Experts located Philae earlier this month, spotting it wedged in a dark crack on the comet.

The one-way Rosetta mission was first considered in the late 1970s but was not approved by the European Space Agency (ESA) until 1993.

The original mission target had been comet 46P/Wirtanen, but it was changed to 67P/Churyumov-Gerasimenko when it was clear that the launch would take place in 2004. The spacecraft launched from Kourou in French Guiana on board an Ariane rocket on 2 March that year.

Why is the mission ending?

Travelling further from the Sun than ever before and faced with a significant reduction in solar power needed to operate, the Rosetta team have decided the spaceship must end it days with Philae on the comet - hopefully within 5km of each other so "mother and child" can be reunited.

Once it's landed - that's it. No more contact with Rosetta and all its equipment will be turned off.

Watch | Philae makes historic landing on comet

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Here's a run-down of its final moments. All times are BST.

September 29 at 9:50pm: Rosetta manoeuvres itself so it's on a collision course with Comet 67P. When it's 19km above the surface it will begin its free-fall and collect its last bits of scientific data.

September 30 at 11:40am: Impact is expected to occur (give or take 20 minutes for error).

September 30 at 12:30pm: Confirmation of impact is expected at mission control due to the 40 minute time-lag between the comet and Earth (give or take 20 minutes for error).

Why is it called Rosetta?

Rosetta takes its name from the Rosetta Stone, a piece of black basalt incised with the same priestly decree concerning Ptolemy V, in three scripts, Egyptian Hieroglyphs, Egyptian Demotic and Greek.

The great significance of the Stone is that it provided the key to deciphering Egyptian hieroglyphs. Scientists hope that the Rosetta mission will unlock the mysteries of how the Solar System evolved.

The Rosetta Stone is currently housed in the British Museum in London.

The Rosetta Stone at the British museum in London.Credit:
Photo: AP

Why did it take 10 years to reach the comet?

The comet is moving far faster than speeds which could ever be achieved by a space ship leaving Earth. So Rosetta needed to use the gravitational pull of the Earth and Mars to act as a sling shot and allow it to pick up acceleration.

When it reached the crucial speed in July 2011 the spacecraft was put into deep-space hibernation for the coldest, most distant leg of the journey as it travelled some 497 million miles from the Sun, close to the orbit of Jupiter as the comet headed into outer Solar System.

Scientists extended its solar arms to catch the Sun’s rays and placed in a slow spin to maintain stability. The only devices left running were its computer and several heaters.

In January 2014 the spacecraft was woken up by an internal alarm clock when it was within 214 million miles of the Sun. It finally reached the comet on August 6 2014. The lander Philae was set down on November 12 2014.

When Philae landed on the comet it was hurtling through space at 24,600 miles per hour and its nucleus was only 2.5 miles wide. Scientists compared the task to a fly trying to land on a speeding bullet.

Why land on a comet?

Comets are the primitive building blocks of the Solar System, left over from a planet-building time when our Sun was just a disc of spinning disc of dust and gas.

Made of ice, dust and small rocky particles, it is likely they delivered the first water to Earth and may have even seeded the planet with the building blocks for life.

Cometary dust brought back to Earth by NASA’s Stardust mission contained glycine, an amino acid that is a basic building block of life.

What is Comet 67P like?

Comet 67P/Churyumov-Gerasimenko is in the Jupiter family of comets. It was thought to be a large, dirty snowball, around 4.4km in diameter, whose orbit around the Sun takes 6.6 years. This makes it a short-period comet.

Images taken suggested that the comet was emitting water vapour into space at about 300 millilitres per second.

Meanwhile, the Visible and Infrared Thermal Imaging Spectrometer, VIRTIS, measured the comet's average temperature to be about -70ºC, indicating that the surface is predominantly dark and dusty rather than clean and icy.

Images taken from Rosetta as it neared the comet showed that the surface was pitch black with no ice on the surface. The nucleus of the comet comprises two distinct segments joined by a 'neck', giving it a duck-like appearance.

Comet 67P/Churyumov-Gerasimenko is so small that its gravitational pull is several hundred thousand times weaker than on Earth.

A close-up picture of Philae.

Who discovered it?

67P/Churyumov-Gerasimenko is named after its discoverers, Klim Churyumov and Svetlana Gerasimenko, astronomers from Kiev who “spotted” the comet for the first time in 1969 on a photographic plate.

The 'P' identifies short-period comets with a well-established orbit around the Sun and that take less than 200 years to complete a solar revolution. The number 67 refers to Churyumov-Gerasimenko's position in the list of catalogued periodic comets. The most famous comet, Halley, is designated 1P.

Where was 67P when Rosetta caught up?

Rosetta met 67P/Churyumov-Gerasimenko when it was still in the cold regions of the Solar System at over 600 million kilometres from the Sun. It was in the middle of its journey back into the inner Solar System.

Did Rosetta land on the comet itself?

No, Rosetta just carried Philae, which is around 35 cubic feet in size. Philae landed on the smaller of the comet's two 'lobes' on November 12, 2014.

Philae is named after an island on the river Nile, where an obelisk was found containing an inscription which played an important role in deciphering the hieroglyphics on the Rosetta stone.

Philae was supposed to fire a harpoon to anchor itself on to the comet after ejecting from Rosetta. It was designed so that it could land on a slope of up to 30 degrees and its three feet were equipped with large pads to allow it to touch down on a soft surface.

Once attached, it should have sent back a panorama of its surroundings and high-resolution pictures of the surface and perform analysis of the composition of the ices and organic material.

A drill was also set to take samples from 8-11 inches below the surface, feeding them to Philae’s laboratory for analysis.

The Philae lander on the surface of comet 67PCredit:
PA

Did everything go to plan?

No, unfortunately Philae’s harpoons did not fire on landing to keep the probe anchored to the surface. It bounced three times before settling under the shadow of a cliff where its solar panels could not pick up enough energy.

Although the probe managed to send back some data , including images of its landing site, it ran out of power just days later, on November 14 2014, and stopped communicating with Rosetta.

Was that the end of Philae?

Pretty much. Luckily the comet was moving closer to the Sun and scientists had hoped that when there was enough sunlight falling on the southern hemisphere that Philae might have enough energy to wake up.

On June 13 2015 the first signal came back from Philae and showed that the probe had been taking samples for months, but did not have enough power to signal. But that was the last time scientists saw or heard from Philae.

However, earlier this month, they were amazed to find the lander wedged in a dark crack on the comet.

What has Rosetta been doing since Philae landed?

Rosetta has stayed alongside the comet analysing the gases of the tail; probing its interior, measuring dust grains and studying its atmosphere and gravity.

The comet reached its closest distance to the Sun on 13 August 2015 at about 115 million miles, roughly between the orbits of Earth and Mars.

How big is Rosetta and Philae?

Rosetta looks like a large aluminium box measuring 2.8 m × 2.1 m × 2.0 m, on which all subsystems and payload equipment are mounted. Two solar panel 'wings', with a combined area of 64 m², each stretch out to 14 m in length. The total span from tip to tip is 32 m.

The Philae lander is smaller, measuring about 1 m × 1 m × 1 m (before deployment of its landing gear) and was carried on the side of Rosetta until it ejected.

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How was the Solar System formed?

The Solar System formed about 5 billion years ago when a cloud of gas and dust – called the ‘pre-solar nebula’ – started to collapse due to gravitational forces.

A disc of leftover material made of the same gas and dust present in the primordial cloud formed around the still-forming Sun. After the Sun ‘ignited’ and began its life as star, most of the particles in this disc collided and stuck to one another, growing in size until they became the planets and the other Solar System bodies.

However, it took some time before the Solar System became the way it is now. About 4.5 billion years ago, it was still 'under construction', and interplanetary space was littered with conglomerates of dust particles. Many of these chunks hit the planets and were destroyed in the collision, but thousands of millions of them survived – they are the asteroids and comets we know today.

How many comets are there in the Solar System?

There are billions of comets in our Solar System, which are typically located in one of two regions.

The most distant repository of comets is the Oort cloud, at the edge of the Solar System, 100,000 times more distant from the Sun than the Earth, and which is estimated to contain about 12 billion comets.

Closer in, just beyond the orbit of Neptune, is the Kuiper belt, which also contains billions of comets and extends from 30 to 50 times the distance equivalent to the Sun-Earth separation (150 million km, or 1 AU).

Some comets escape from these regions and journey into the inner Solar System. Every year many new comets are discovered in this region, often ‘sungrazers’ spotted by ESA/NASA’s SOHO spacecraft. Sungrazers travel very close to the Sun, and are sometimes partially or completely destroyed in the encounter. Comet ISON is a well-known example of a sungrazing comet.

What is the difference between asteroids and comets?

Comets are typically nicknamed 'dirty ice-balls'. Asteroids, or minor planets, are known in very simple term as ‘rocks in space’.

The size of asteroids typically ranges from a metre to several hundred kilometres across.

One of the main differences is that asteroids do not usually contain ‘volatiles’ (substances that sublimate i.e. when heated they pass directly from the solid to the gaseous state). Therefore asteroids do not develop a tail or a coma when they approach the Sun.

However, a recent class of object discovered in the main asteroid belt reveals that some asteroids behave like comets, sometimes suddenly sporting a dust tail. These are termed ‘main belt comets’.

There is also good evidence that some asteroids are 'dead comets', comets that have lost their volatile materials after many approaches to the Sun.

How has Rosetta differed to other comet missions?

Rosetta is a much more ambitious and advanced mission than previous comet missions like Giotto, Stardust or Deep Impact. Its observation phase lasted much longer and was not limited to “snap-shots” from flybys.

Giotto obtained a mass of new information but its period of observation was limited to two short-lived flybys.

Stardust captured some excellent black and white images and gathered samples of dust from the comet’s coma. However, it was not designed to provide information on the nature of the nucleus.

Deep Impact filled in this missing gap but its instrument package included only cameras and a single infrared spectrometer, so most analysis of the comet’s composition had to be done from the ground.

Unlike these missions, Rosetta included both an orbiter and a lander and has been capable of investigating both the nucleus and the coma over a long period of time.

It has also carried a much more advanced payload than any of its predecessors. The suite of eleven experiments on the orbiter have observed all aspects of the comet from close range as it moves along its orbit towards the inner Solar System, permitting scientists to study the composition of the coma and nucleus in great detail.

For example, they have been able to examine parent molecules on the comet’s surface that originated from the nucleus that have not yet been modified by the space environment, and survey the complex physical and chemical changes in the nucleus as it is warmed by the Sun.

The experiments on the lander, including spectrometers, high-resolution cameras and drill, will permit a more detailed comet investigation than has ever been done before.